Fat-Alfie points out (in the Oschersleben comments) that the SR3 'borrows' the big fat tires of the Praga R1. But it isn't just the grip that makes Shaun's Radical such a great drive, it's the overall balance that hits the Goldilocks 'sweet spot.' The steering is precise, the suspension is compliant (spoiler: except it doesn't handle kerbs all that well), the (fixed) gear ratios are well chosen, the brakes are effective and it responds well to setup tweaks. The cockpit load is agreeable...
Radical SR3 @ Laguna Seca
Radical SR3 @ Laguna Seca